Wireless communication systems are known to comprise mobile units (i.e., hand-held portable or in-car mobile radiotelephones) that wirelessly communicate digitally-represented voice signals within sites controlled by site controllers. Often, site controllers communicate with a switching center so that multiple sites can communicate with each other via the switching center. Further still, sites can communicate with each other via two or more switching centers that are in communication with one another. Current switching centers typically route voice signals in non-compressed digital voice formats, e.g., a pulse-code modulation (PCM) format. In contrast, mobile units typically transmit and receive voice signals in the form of compressed digital voice, e.g., data packets created through the use of a vector-sum excited linear predictive (VSELP) voice coder or an Improved Multiband Excitation (IMBE) voice coder.
Transcoders are provided to perform the conversions, when required, between the compressed digital format and the non-compressed digital format and vice versa. A typical example occurs when a land-based telephone, using a non-compressed digital format, communicates with a mobile unit via the switching center. In this situation, compressed digital voice from the mobile unit is converted into the non-compressed digital format by a transcoder associated with the appropriate site controller. The non-compressed digital voice is then routed, via the switching center, to the land-based telephone. When communicating from the telephone to the mobile unit, the process is reversed.
Another situation in which transcoders are used occurs in calls between two mobile units. In this case, compressed digital voice from a first mobile is converted to a non-compressed format by a transcoder and sent through a site controller to a switching center. The switching center, in turn, routes the non-compressed voice possibly to the same site controller or another site controller, which re-converts the non-compressed voice, via another transcoder, to the compressed format. The re-compressed digital voice is then transmitted to the second mobile via the site controller. As known in the art, conversions by a transcoder add delay to the transmission of the message and slightly degrade the resulting quality of the voice. Thus, calls requiring more than one conversion, as in the present example, may be subjected to prohibitive delays and audio degradation.
An attractive solution to this problem is to allow compressed digital voice signals to be transparently passed through the transcoders, and hence through the switching center, when a mobile-to-mobile call is established. In this manner, the delays and audio degradation inherent in the conversion process can be avoided. Transcoders capable of operating in either a transcoding mode (i.e., converting compressed digital voice to non-compressed digital voice) and a transparent mode (i.e., passing compressed digital voice without conversion) are discussed in greater detail in U.S. patent application Ser. No. 08/336,347 filed Nov. 8, 1994 now U.S. Pat. No. 5,608,779, and assigned to the same assignee as the present invention. Assuming that transcoders can operate in this manner, i.e., transparently passing compressed voice, current switching center technology does not provide the necessary control for such operations.
The problem described above is exacerbated when voice messages are required not only to span multiple transcoders, but also to span multiple switching centers. Even if the transcoders affiliated with each switching center are capable of passing compressed digital voice, operations associated with the inter-switching center communication link must be similarly controlled. For example, the echo-cancellation found on most inter-switching center communication links would need to be disabled when passing compressed digital voice. Current methods for controlling the operations of switching centers do not address the need to pass compressed voice between switching centers, i.e., a mobile-to-mobile call is treated similarly to any other type of call. Therefore, a need exists for a method that avoids multiple transcoder format conversions for mobile-to-mobile calls that span multiple switching centers. Such a method should take advantage of opportunities to use reduced-bandwidth resources when available.